The present embodiments relate to dynamic medical imaging systems.
Magnetic resonance imaging (MRI) and computed tomography (CT) are medical imaging techniques in widespread use for viewing the structure and function of the human body. MRI systems provide soft-tissue contrast, which may be useful for diagnosing soft-tissue disorders, such as tumors. CT is an image processing technique that generates a three-dimensional (3D) image from a large number of two-dimensional (2D) X-ray projections.
Radiation therapy, or radiotherapy, has relied on MRI and CT to direct radiation at malignant tumors as part of cancer treatments. The information provided by the images helps determine a 3D dose distribution and avoid damage to surrounding tissue. Unfortunately, radiotherapy is hampered by motion and deformation of the organs in the thoracic and abdominal cavities, e.g., the lungs, liver and pancreas, during respiration. Such organ motion may lead to a discrepancy between planned and actual target positions. Until recently, there were no readily available imaging modalities capable of resolving and rendering respiratory motion in four dimensions, the fourth dimension being time. A typical strategy to account for respiratory motion used conventional CT, along with a generic uncertainty margin, to define a target volume for radiotherapy treatments delivered with patients breathing freely. A limitation of this technique is that the uncertainty margins are typically conservative (overly large and non-patient-specific), which leads to sub-optimal target conformality and thus more radiation dose to normal tissues.
More recently, radiotherapy treatments have become increasingly sophisticated with better imaging and targeting methods. In so called “respiratory gating” techniques, respiration is monitored during imaging and treatment. One such technique acquires or retrospectively samples images at a particular portion of the respiratory cycle, e.g., typically near end of normal exhalation. The same “gating window” is then used for the treatment delivery. By acquiring CT images or slices throughout a full respiratory cycle at each nominal slice location, time-resolved, or four-dimensional (4D), X-ray computed tomography (“respiratory-correlated CT” or “4D-CT”) images are derived. 4D-CT images have been used to define patient-specific target volumes in connection with lung and abdominal radiotherapy and for dose planning.
4D-CT typically samples a single respiratory cycle at each slice location. As a result, slice-to-slice and phase-to-phase consistency of the 4D-CT image reconstruction relies on anatomic reproducibility in the breathing cycle. Irregular breathing leads to volume inconsistencies and geometric reconstruction artifacts. Furthermore, temporal limitations in CT image acquisition (mainly due to restrictions of gantry speed) oftentimes result in intra-phase residual motion, which tends to increase apparent volumes of tumors and contextual anatomy around the inhale (higher-velocity) portion of the respiratory cycle. Attempts to address these limitations with repeated 4D-CT acquisition may expose the patient to increased levels of ionizing radiation.
Dynamic MRI has been used to resolve respiratory motion. MRI procedures are not limited by concerns regarding radiation exposure. Dynamic MRI sequences can provide higher effective frame rates than 4D-CT, minimizing residual intra-phase motion. However, dynamic 3D MRI acquisitions are not yet suitable given present limitations in either signal-to-noise ratio (SNR), spatial resolution, or frame rate.
2D dynamic MRI images have also been used to resolve respiratory motion. For example, 2D slices have been stacked to generate a time-resolved, 3D liver volume. The volumes are retrospectively sorted by searching for similar anatomic liver states. See von Siebenthal et al., “4D MR imaging of respiratory organ motion and its variability,” Phys. Med. Biol., Vol. 52, pp. 1547-1564 (2007). This technique may suffer from lack of extendibility to other sites of disease, and from a lower effective frame rate.